1. Field of the Invention
The invention concerns electrostatic paint spraying apparatus. The word "paint" as used in this specification and claims should be interpreted broadly as covering any coating material to be sprayed on to the surface of a workpiece or object, for whatever purpose e.g. protection or decoration.
2. Description of the Prior Art
In the field of electrostatic paint spraying a variety of spraying heads are known. Rotary spraying heads may be shaped as a bell, mushroom, dish or disc. A common characteristic of these rotary devices is that the paint is atomized partially by the centrifugal force and partially by electrostatic forces. In the electrostatic force field the paint is formed into a film at an appropriately shaped atomizing edge and the thin film of paint leaving the edge is transformed into a mist of very fine droplets.
Known spraying devices may be made of metal or plastics. The spraying edge of a metallic spraying device is generally made sharp and is connected to high voltage current source the other pole of which is grounded. Thus the metallic spraying device has a very high potential relative to ground, e.g. 50 to 150 kV. The object to be coated is also ground and positioned in front of the spraying device at a distance of about 250-400 mm to form an extremely strong electric force field with the spraying device connected to high voltage. The average field intensity is given by the ratio of the potential difference and the distance which thus in the case of 100 kV and 300 mm is 3.3 kV/cm. However, this value of the field intensity is considerably enhanced at the sharp edge of the spraying device as a consequence of the so-called electric corona effect and can exceed the breakdown potential of air. Thus, at the atomising edge the electric charge flows into the air accompanied by a corona discharge. If paint is present at this edge, then the very high field intensity causes the paint to form into a ribbon or filament which then is transformed from droplets into a standing paint cloud in the manner already described.
Where the spraying device is made of plastics, then the paint layer itself forms the conductive or semi-conducting layer which conveys the charge to the edge of the spraying device. There the ribbon and cloud formation takes place in a manner similar to that already described. With plastics spraying devices, especially where the resistance of the paint to be sprayed is high, the field strength at the edge of the surface is smaller and thus the efficiency of spraying is reduced. The efficiency of a plastics spraying device depends more on the specific resistivity of the paint employed i.e with such devices only paints in a narrower resistivity range can be sprayed.
Electrostatic spraying devices are also known which rotate at very high speed, of the order of 20,000-30,000 r.p.m., and where spraying takes place exclusively under the effect of the high centrifugal forces while the electrostatic charge is imparted to the spray mist indirectly and only after separation from the rotary edge.
Another known method of electrostatic spray painting is the so-called auxiliary electrode system wherein the high electric field strength necessary for charging the paint is produced by a main and auxiliary electrode connected to opposite polarities and disposed relatively close to each other.
An advanced type of system is a so-called capacitor type electrode arrangement wherein the paint is in direct contact with the main electrode and a solid dielectric layer is between the auxiliary electrode and the paint, see U.S. Pat. No. 3,735,925 (=GB-PS No. 1,335,071=DT-PS No. 2,059,594 and others). Because of the small distance between the electrodes in this arrangement, extremely high field strengths can be achieved in spite of the use of relatively small voltages and thus in this system high electric charges can be imparted to the paint cloud with operating voltages of only 15-30 kV, i.e. smaller than conventional voltages.
While this system is suitable for various kinds of electrostatic spraying devices it has been used primarily with air atomizing devices. This is because air atomization imparts to the paint cloud a characteristic initial velocity and direction of advance which causes the paint cloud to pass into the vicinity of the object to be coated where the charge carried by the paint droplets is in itself sufficient for deposition on the surface of the object.
In this system, the atomized paint cloud has a very high charge. However, any insulated or grounded object or metallic body attracts the electrically charge paint cloud and thus it is not only the object to be coated that attracts the paint particles but also the counter-electrode of the capacitor system, which is also grounded. Where atomization takes place primarily under the effect of electrostatic forces and the paint cloud leaves the spraying device at a relatively low speed only, the vicinity of the grounded electrode and its charge of opposite polarity exerts a very strong attraction on the cloud and a part thereof is thus deposited on the surface of the insulating layer covering the electrode. Thus, a part of the atomized paint is deposited back on the spraying device.